Drilled shaft construction often requires the use of drill slurry to maintain borehole stability during excavation and concreting. Florida Department of Transportation (FDOT) specifications require that the mineral slurry used for all primary structures must stay within viscosity limits of 30 and 50 sec/qt. This study addressed the rationale for defining the uppermost limit. The lower limit was previously investigated. Two types of tests were undertaken to assess the effect of the upper viscosity limit on shaft performance. These tests, included: rebar pullout tests and side shear tests where the presence of slurry may impede the structural and geotechnical capacities, respectively. The results of these tests concluded that the presence of any bentonite slurry at the time of concreting reduces the rebar bond. Reductions ranged from 25 to 70% for slurry viscosity of 30 to 90 sec/qt, respectively. Similar tests with polymer slurry showed smaller reductions. The effect on side shear was evaluated at both model and full scales. Model tests conducted over a wide range of viscosity showed only modest reductions in side shear for shafts cast with 50 sec/qt slurry when compared to shafts cast in 40 sec/qt slurry. However, shafts cast with 90 sec/qt slurry showed a marked increase in side shear. Full-scale tests were conducted with both mineral and polymer slurry where four different slurries were used (average viscosities were: 40 sec/qt and 74 sec/qt bentonite as well as 50 sec/qt and 131 sec/qt polymer). Those tests showed use of higher viscosity bentonite or polymer slurry had no adverse effects on side shear (again compared to 40 sec/qt). In fact, increases in the unit side shear resistance of 19%, 12%, and 13% were recorded for 74 sec/qt bentonite, 50 sec/qt and 131 sec/qt polymer, respectively. Finally, physical inspection of shafts with congested cages (6 in. clear space) cast in bentonite slurry indicated that there may exist potential durability issues as pathways/creases of trapped bentonite formed at all rebar locations that extended from the rebar out to the soil/concrete interface. This was most pronounced for slurry with viscosity greater than 40 sec/qt.

Defining the Upper Viscosity Limit for Mineral Slurries Used in Drilled Shaft Construction

A Theoretical and Practical Analysis of the Effect of Drilling Fluid on Rebar Bond Strength

Drilling slurry made from a mixture of water and mineral or polymer powder is often used to stabilize deep excavations, in which cast-in-place reinforced concrete foundation elements such as drilled shafts are constructed. At the time of concreting, drilling slurry is displaced by heavier, highly fluid concrete tremie placed at the bottom of the excavation from within the reinforcement cage. Despite the fluidity of the concrete, it must build up within the reinforcing cage to a sufficient height before then radially pressing into the annular cover region. This flow pattern has been shown to trap slurry near the steel reinforcement and affect reinforcing bar bond. This paper presents the results of 227 reinforcing bar pullout tests performed over a 6-year period from 2012 to 2018. Shaft specimens were tremie placed in various slurry types and consistencies. This analysis concluded that present development length values should be increased by 1.8 for bentonite and 1.9 for polymer slurry; water casting environments showed no significant change relative to dry conditions.

Reinforcing Bar Pullout Bond in Tremie-Placed Concrete Cast in Drilling Slurry Environments

This report presents results from a two-year experimental study to evaluate the interface bond between a cast-in-place seal slab and prestressed concrete or steel piles. Both scale model and full-scale tests were conducted and several cofferdam conditions were simulated. These were (1) marine environment, (2) fresh water conditions, and (3) drilling fluid condition. Normal pile surfaces were investigated. Additionally, the situation of "soil-caked" piles was also investigated. In the model tests, a total of 36 one-third scale specimens were tested - 28 prestressed concrete and 8 steel. Bonded embedment depth in the seal slab was varied between d to 2d where d was the size of the pile. The results of these tests indicated that shear stress variation was non-uniform leading to larger computed bond stresses with shallower embedment. Values were least for drilling fluid. Concrete piles had better bond with the seal concrete than steel piles. "Soil-caked" condition was found to be relevant for the drilling fluid situation only. In other cases, it was washed away from the pile surface. In the full-scale tests, a total of 32 specimens were tested divided equally between steel and concrete. The prestressed piles were 14 in. square and the steel piles were 14 in. deep wide flange sections. Embedment depth was varied between 0.5d to 2d, i.e., 7 to 28 in., with the larger depth reserved for the drilling fluid condition. Four of the 16 prestressed piles were cast with embedded gages located at the top, middle and bottom of the interface region. The results of the full-scale tests were similar to those from the one-third scale tests. The most important findings were (1) loads were transferred over a distance equal to the depth of the pile, (2) scale effects were present - the average calculated bond stresses were lower for the full-scale tests than from the corresponding scale model tests, (3) prestressed piles cracked prior to bond failure, and (4) the seal slab cracked prior to bond failure. Based on the test results it is proposed that the interface bond between piles and the seal slab be restricted to an effective area in contact with the cast-in-place seal slab. The effective area is calculated using the actual embedment depth or the size of the pile, whichever is smaller. The average bond stress over this region is limited to 300 psi for concrete piles and 150 psi for steel piles. These values are reduced by a third in cases where drilling fluid is used. Application of the proposed values to the conditions related to the full-scale tests led to average factors of safety in excess of two for both the prestressed and steel piles. However, tension loads taken by the piles should not lead to cracking (concrete) or exceed the allowable tension load (steel) of the piles. Nor should the seal slab crack.

R Sosa

Papers

Seal slab/pile interface bond

Full-Scale Testing of Seal Slab/Pile Interface Bond